Microwave level gauge, or radar level gauge (RLG), systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a guided wave transmission line.
In a GWR-system, a probe is generally arranged to extend vertically from the top of the tank towards the bottom of the tank. Probes for GWR level gauge systems are typically provided in the form of wire probes or rigid probes. A wire probe may comprise a metallic wire having a diameter between a few mm up to tens of millimeters. Rigid probes often have a larger diameter.
In level gauge system used in large tanks, such as tanks for liquid natural gas (LNG) which may be around 30 m high, wire probes are often used due to the ease of installation. Installation of a wire probe may comprise attaching a weight to the bottom of the probe and simply lowering the probe into the tank.
However, for certain applications it may be desirable, or even necessary, to use a rigid probe. A rigid probe is typically more robust and thereby more resilient to both mechanical wear and tear as well as to chemical exposure. As is readily realized, a conventional rigid probe for use in large tanks formed in one piece can be difficult to both transport and install.
Furthermore, the varying size of tanks for different applications means that a level gauge system must be provided with a probe where the length is either tailor made for the specific tank, where the length may be adjusted during installation.
To overcome the problems associated with using rigid probes in large tanks, U.S. Pat. No. 7,012,437 discloses a fill level measuring device where the signals are guided along a surface waveguide, and where the surface waveguide comprises a plurality of segments which are arranged to telescope into, and out of, each other. Thereby, a probe having an adjustable length is provided.
However, as the probe disclosed by U.S. Pat. No. 7,012,437 comprises telescoping segments, there is a change in diameter of the probe at each transition from one segment to the next. Such diameter transitions may disturb the signal being guided along the probe and introduce spurious eco signals on the probe.
Furthermore the length of the probe is controlled by a regulator control unit, thereby adding components and complexity of the system, which in turn increase the overall cost of the measuring device.